Document DE 10 2005 002 337 A1 makes known a transmission device, which is designed as an 8-speed, multi-stage transmission having friction-locking shift elements, such as multi-disk clutches and multi-disk brakes. In the event of a shift request for a transmission ratio change in the transmission device, at least one friction-locking shift element—which is engaged in the power flow of the transmission device in order to implement the actual transmission ratio currently engaged in the transmission device—must be disengaged from the power flow of the transmission device, while at least one further friction-locking shift element—which is disengaged from the power flow of the transmission device during the implementation of the actual transmission ratio currently engaged in the transmission device—must be engaged into the power flow of the transmission device in order to implement the requested actual transmission ratio.
Therein, the torque that is transferred via the friction-locking shift element engaged in the power flow in order to implement the current actual transmission ratio of the transmission device is transferred—more or less, as the shift time increases—from the friction-locking shift element to be engaged into the power flow of the transmission device in order to implement the requested target transmission ratio, while the torque that can be transferred via the shift element to be disengaged decreases.
In general, in the event of a request to disengage a friction-locking shift element independently of the torque currently transferred via the friction-locking shift element, it can be assumed that the friction-locking shift element actually transitions into the disengaged operating state, which is why a simple software-based evaluation of a control current of a pressure controller of a friction-locking switching element is sufficient. Upon detection of a corresponding disengagement control signal for transferring a friction-locking shift element to the disengaged operating state, it is possible to easily verify, by means of the aforementioned evaluation, whether a shift or gear-disengagement procedure that includes disengagement of a friction-locking shift element was successful. Given a suitable design of the components of a friction-locking shift element, for example a spring preload, friction-locking shift elements also actually transition into the disengaged operating state with a high degree of probability, provided the pressure controller of the friction-locking shift elements is actuated correctly with a disengagement control signal.
Disadvantageously, friction-locking shift elements in the disengaged operating state cause drag torques, however, which lower an overall efficiency of an automatic transmission to an unwanted extent.
For this reason, transmission devices such as that known from DE 10 2008 000 429 A1, are also being designed to an increasing extent not only with friction-locking shift elements but also with form-locking shift elements, wherein no drag torques occur in the region of these form-locking shift elements in the disengaged operating state that can lower the overall efficiency of a transmission device.
It must be taken into account, however, that form-locking shift elements, as compared to friction-locking shift elements, must be close to the synchronization point in order to be switched from a disengaged operating state, in which torque cannot be transferred via the form-locking shift elements, into the engaged operating state, in which all the available torque can be transferred via the form-locking shift elements. In addition, form-locking shift elements engaged in the power flow of a transmission device must be close to the load-free operating state in order to be disengaged from the power flow with low shifting forces. During shift procedures or so-called gear-disengagement procedures, a power flow between a transmission input shaft and a transmission output shaft must be interrupted in the region of a transmission device. Therein, a form-locking shift element must be transferred from the engaged operating state to the disengaged operating state. Under certain circumstances, the form-locking shift element cannot be transferred to the disengaged operating state due to an excessively rapid or faulty build-up of the torque present in the region of the form-locking shift element, for example during an upshift or a faulty reduction of the torque that is present, for example, during a gear disengagement. It is also possible for mechanical, hydraulic, or electrical malfunctions to prevent the disengagement of a form-locking shift element, which is why, if a corresponding disengagement control signal is present, form-locking shift elements, as opposed to friction-locking shift elements, do not necessarily transition into a disengaged operating state and a gear set of a transmission device is in an overdetermined operating state, which is unwanted, or a requested interruption of the power flow cannot be implemented in the region of a transmission device.